1. Field of the Invention
The present invention relates to an apparatus for locating a moving carrier, such as an originals carrier or a lens carrier used in a process camera unit, exactly at a desired position, wherein the apparatus includes a combination of a driving motor and an incremental encoder, such as a rotary encoder.
2. Description of the Prior Art
To locate a moving carrier precisely at a desired position, a stepping motor is commonly used. However the highly complicated circuit in the driving system of the stepping motor is a problem, as is the high cost thereof. To solve these problems an encoder, such as a rotary encoder, is usually employed. However when an encoder is used, it is unavoidable that the carrier moves a little further by inertia after the motor is deenergized, wherein the inertia occurs in the driving system including the motor.
In order to locate the carrier precisely at a desired position, the following methods can be generally adopted:
(1) The motor is electrically or mechanically stopped as soon as the number of pulses generated by the encoder reaches a predetermined value; PA1 (2) When an induction motor is used as a motor, the speed of the motor is varied step by step. When a servomotor is used, the speed is continuously varied, and is reduced to a lowest possible value immediately before the number of pulses from the encoder reaches a predetermined value. When the number of pulses reaches the predetermined value, the driving signal to the motor is stopped; or PA1 (3) Every time the carrier passes a predetermined reference point, the counting of pulses is reset to a predetermined value, and accumulating errors are automatically corrected. This system is disclosed in Japanese Patent Publication (examined) No. 57 (1982)-13859, and is used in combination with either the above-mentioned (1) or (2). PA1 a shifting means for shifting a carrier in a desired direction; PA1 a driving means for driving the shifting means; PA1 position generating means operable in association with the shifting means; PA1 an input means for inputting a reference position signal, the means being located within the range of movement of the carrier; PA1 a target setting means; PA1 an arithmetic operation unit; and PA1 a motor driving circuit; PA1 wherein the arithmetic operation unit comprises a first memory for storing current position data of the carrier, the first memory being adapted to count pulses from the position signal generating means, a second memory for storing data corresponding to the rotations of the driving motor, a third memory for storing data of the inertia acting upon the carrier, and an arithmetic operator, whereby a driving quantity of the carrier is calculated on the basis of a target data from the target setting means, the current position data and the inertia data, the motor being driven based on the information of the driving quantity so as to move the carrier toward the target, and the current position data is corrected, or updated in accordance with the position taken by the carrier.
The method mentioned in (1) requires an electrical or mechanical controller, and under this method it is unavoidable that the carrier is liable to shocks when it is braked. The method mentioned in (2) also requires a speed controller, which is likely to be complicated and expensive. This is a disadvantage.
A disadvantage common to the three prior methods is the difficulty of equalizing the dimensions, shapes and sizes of the components. The unequal components lead to the different movement of the carrier under inertia, which means that the movement of the carrier by inertia differs from apparatus to apparatus. In addition, variations in movement caused by inertia occurs in each apparatus over a long period of use. As a result the apparatus can not maintain its efficiency for a long time.